Not sure if this should be here but anyway..I was looking through Openstreetmap when i noticed off the Antrim coast in the Irish sea there is a small area of sea which is surrounded by an International Border .Im pretty sure this must be a mistake or is there some unknown independent island here ..maybe atlantis ? http://www.openstreetmap.org/

It most likely is International waters. Bit of an oddity considering how small of an area it is. No disputed zones up that way ?

Yes, International waters it is.
OpenStreetMap can often show up some real oddities in international borders - have a look at Bangladesh's northern border with India: http://osm.org/go/zrs3SAT-- Largest number of enclaves and counter-enclaves in any border worldwide.

As larryone says, this is technically recorded as (see below) international waters. As you will see, the marine borders in OSM are mostly set at the default buffer distance from the coast as laid down by international treaties.

Actual territorial waters can differ in accordance with specific treaties, and where an OSM mapper is aware of this and could be bothered, the OSM boundaries may reflect this. So in the instance of this little triangle, this may indeed reflect an island of international waters surrounded by UK waters or it might in fact be accepted internationally as UK waters but OSM doesn't yet know it.

I think those borders (and loads of borders) on Open Street Map were automatically created by someone based on the '12 nautical miles from the coast' rule of thumb (or whatever it is). So that little triangle is not really international border, but just an area 12 n.m. from land.

Borders are often disputed, especially maritime borders. There was a time when Ireland half seriously claimed all the sea around Northern Ireland.

If you have a matter that involves international waters and maritime law, consult a lawyer.

There was a time when Ireland half seriously claimed all the sea around Northern Ireland.

Was that claim ever revoked? Certainly, the best information I can find suggests that there is still no agreed border through Carlingford Lough and Lough Foyle (a fact that caused a brief edit war on OpenStreetMap a while back).

Was that claim ever revoked? Certainly, the best information I can find suggests that there is still no agreed border through Carlingford Lough and Lough Foyle (a fact that caused a brief edit war on OpenStreetMap a while back).